Summary:
In this study the authors goal try discover relationships between design context of classes and the amount of changes applied to them during maintenance. Four software systems are studied starting from their initial versions and analyzing changes made in each subsequent version. Two of these are commercial systems, while the other two are open source projects. The results are compared against a previous study that analyzed only one piece of software. Similar hypotheses are made as in the previous study: Larger classes are more change prone, classes that participate in design patterns don't change as much as other classes.

The results disprove the first hypotheses, the graphs indicate that the first three systems analyzed do not show any relationship between size and number of changes. Only one open source system indicates a somewhat strong indication that larger size causes more changes. This is opposite of the results obtained in the previous study. Such results suggest that class size does not predict changes. The second hypotheses is supported by the results, which is consistent with the previous study. Classes that were part of patterns were more prone to changes in three of the four systems. However one open source system had the opposite results.

I think that the manual collection of patterns can be a big threat, since developers are not very likely to document design patterns. However, I liked that this study analyzed both commercial and open source software which I think increases its external validity, since conditions for development and maintenance in the two types of software are different. Also I liked the large variety of design patterns that appeared in studied software. I agree with the authors that more research needs to be done into the cause of the changes, since these could be the main determining factors in how the software is changed.